In the course of manufacturing paper and similar products, including paperboard and the like, it is well known to incorporate quantities of inorganic materials into the fibrous web in order to improve the quality of the resulting product. In the absence of such "fillers", the resultant paper can have a relatively poor texture due to discontinuities in the fibrous web. The fillers are also important in improving the printing qualities of the paper, i.e., by improving the surface characteristics. The use of appropriate fillers additionally improves the opacity and the brightness of a paper sheet of a given weight.
A number of inorganic materials have long been known to be effective for many of the aforementioned purposes. Among the best of these materials is titanium dioxide, which can be incorporated into the paper in the form of anatase or rutile. Titanium dioxide, however, is among the most expensive materials for such purposes. Thus, despite the effectiveness of such a material as a filler, its use is limited, and satisfactory replacements have been sought.
Among the materials which have found increasing acceptance as paper fillers are calcined kaolin clays. Materials of this type are generally prepared by partially or fully calcining a crude kaolin clay, which may have been initially subjected to prior beneficiation steps in order to remove certain impurities, e.g., for the purpose of improving brightness in the ultimate product. Such products are described in, for example, U.S. Pat. Nos. 3,014,836 to Proctor, Jr., and 3,586,523 to Fanselow et al. as well as in McConnell et al, U.S. Pat. No. 4,381,948.
Those properties which render a calcined kaolin pigment particularly valuable for use as a filler are well known. The brightness and opacifying characteristics are important in producing an acceptable paper sheet, i.e., a sheet which has whiteness, high opacity, good printability and light weight. Additionally, low abrasiveness (i.e. a low abrasion value as measured by standard tests) is significant in order to assure that the resultant paper product may be manufactured and processed using conventional machinery without damaging the machinery.
It is important for an understanding of the present invention, to recognize that those skilled in the art of kaolin processing draw a sharp and fundamental distinction between uncalcined and calcined kaolins. With respect to terminology, it is noted that the prior art literature, including numerous of the prior art patents relating to the field of kaolin products and processing, often uses the term "hydrous" to refer to a kaolin which has not been subjected to calcination--more specifically, which has not been subjected to temperatures above about 450.degree. C., which temperatures serve to alter the basic crystal structure of kaolin. These so-called "hydrous" clays may have been produced from crude kaolins, which have been subjected to beneficiation, as, for example, to froth flotation, to magnetic separation, to mechanical delamination, grinding, or similar comminution, but not to the mentioned heating as would impair the crystal structure.
In an accurate technical sense, the description of these materials as "hydrous" is, however, incorrect. More specifically, there is no molecular water actually present in the kaolinite structure. Thus, although the composition can be (and often is) arbitrarily written in the form 2H.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2, it is now well-known that kaolinite is an aluminum hydroxide silicate of approximate composition Al.sub.2 (OH).sub.4 Si.sub.2 O.sub.5 (which equates to the hydrated formula just cited). Once the kaolin is subjected to calcination, which, for the purposes of this specification means being subjected to heating of 450.degree. C. or higher for a period which eliminates the hydroxyl groups, the crystalline structure of the kaolinite is destroyed. As used in this specification, the term "calcined kaolin" shall refer to such a kaolin. Preferably the calcined kaolin has been heated above the 980.degree. C. exotherm, and therefore is "fully calcined", as opposed to having been rendered merely a "metakaolin". Reference may be had in the foregoing connection to Proctor. U.S. Pat. Nos. 3,014,836 and to Fanselow et al. 3,586,823, which disclosures are representative of portions of the prior art pertinent to fully calcined kaolins.
A calcined product having characteristics generally superior to previously available such calcined kaolin pigments, is the ALPHATEX.RTM. product of E.C.C. America Inc., assignee of the present application. This product again is a substantially anhydrous white kaolin clay pigment, which has unusual efficacy as a filler in paper sheets and similar paper products. The pigment also has application as a coating pigment for paper, and as a pigment in paints and other filled systems. It generally consists of aggregates of anhydrous kaolin clay particles, and exhibits exceptionally high lightscatter and opacifying characteristics when incorporated as a filler in paper.
ALPHATEX.RTM. is further described in U.S. Pat. No. 4,381,948 to A.D. McConnell et al. as being an anhydrous white kaolin clay pigment having high light scattering when incorporated as a filler in paper, the pigment consisting of porous aggregates from sub-micron sized kaolin clay platelets obtained by classification of a dispersed kaolin clay to a 100% less than one micron ESD fraction, the aggregates having an average specific gravity in the range of 0.5 to 0.6 and a mean internal pore size of less than 0.55 microns. The size distribution of the aggregates is such that no more than 5% by weight thereof are greater than 10 microns ESD, at least 75% are of less than 2 microns ESD, and not more than 15% by weight are of less than 1 micron ESD. The pigment has a Valley abrasion value below 30 mg., and a G.E. brightness of at least 93.
There is a continuing search in the kaolin industry for methods for decreasing the abrasion of a calcined kaolin pigment. It is highly desirable that when a method for improving such a property is discovered that it not detrimentally affect another property. For example, if a method is derived for reducing the abrasion of a calcined kaolin pigment, it is highly desirable that such method not materially affect the brightness or opacifying characteristics of the kaolin.
From time to time various methodology has been reported for reducing the abrasiveness of calcium clays by means of additives and the like. One such example is disclosed in the present assignee's, U.S. Pat. No. 4,830,673 to Jones et al. Reduction of abrasiveness is effected therein by contacting the clay prior to calcination with an aliphatic diol, such as ethylene glycol. This process is effective but glycols are relatively expensive materials and it is desirable to improve the economics of abrasiveness reduction as well as to provide an effective alternative method.